The invention relates to a process for the preparation of strongly basic anionic exchangers based on cross-linked vinylaromatic polymers by reacting weakly basic anion exchangers bearing tertiary amino groups with ethylene oxide under defined conditions.
Weakly basic anion exchangers are water-insoluble polymers which contain primary and/or secondary and/or tertiary amino groups, the anion exchangers containing primary amino groups being able to serve as starting materials for the preparation of the anion exchangers containing secondary or tertiary amino groups or containing quaternary ammonium groups.
Strongly basic anion exchangers contain quaternary ammonium groups, preferably those of the type I: 
or of the type II 
where X in each case denotes an anion, such as hydroxyl, chloride, bromide, iodide, fluoride, sulfate, sulfide, hydrogensulfate, hydrogensulfide, phosphate, di-phosphate, mono-phosphate, carbonate, hydrogene carbonate, citrate, tartrate, phthalate.
The two types differ in basicity and as is also otherwise customary in ion exchanger technology, in the case of anion exchangers, depending on the type of object to be fulfilled, the resin is selected according to its basicity. Whereas the anion exchangers of type I can be synthesized by reacting resins bearing primary amino groups with conventional alkylating agents such as methyl chloride, those of type II are obtained by reacting resins bearing tertiary amino groups with chloroethanol. A reaction of this type II is described, for example, in Example 6 of DE-B 1 054 715: there, ethylene oxide is passed at 30xc2x0 C. through a mixture of dilute sulphuric acid and-a weakly basic anion exchanger bearing dimethylamino groups.
However, it has been found that although these strongly basic anion exchangers fulfil their assigned task, impurities which originate from their preparation or are formed during use are released to their environment. The object of the invention was therefore to provide strongly basic anion exchangers of the type II which are superior in this respect to the ion exchangers of the prior art.
In the past, attempts have been made to remove compounds formed in the preparation of the ion exchangersxe2x80x94whether they be unreacted starting materials or low-molecular-weight non-cross-linked polymersxe2x80x94by repeated washing with water, which is complex and only partially successful.
Surprisingly, it has now been found that superior strongly basic anion exchangers of the type II are formed if, during the quaternization, certain temperature and pH conditions are employed.
The invention therefore relates to a process for the preparation of strongly basic anion exchangers based on cross-linked vinylaromatic polymers by reacting weakly basic anion exchangers bearing tertiary amino groups with ethylene oxide, characterized in that the weakly basic anion exchangers are reacted with ethylene oxide at 70 to 75xc2x0 C. and at a pH of 7 to 11, preferably 8 to 10.
The vinylaromatic polymers underlying the ion exchangers are polymers based on vinylaromatics such as styrene, vinyltoluene, ethylstyrene, xcex1-methylstyrene, chlorostyrene, o-chloromethylstyrene, m-chloromethylstyrene, p-chloromethylstyrene, vinylpyridine, vinylnaphthalene. Use can also be made of polymers in whose preparation nonaromatic monomers containing one copolymerizable double bond per molecule are used conjointly. Such monomers include, for example, acrylic acid, methacrylic acid, acrylates, methacrylates, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl chloride, vinylidene chloride, vinyl acetate. The monoethylenically unsaturated monomers for the preparation of the vinylaromatic polymers preferably contain no more than 10% by weight of these nonaromatic monomers, based on the total weight of the monomers.
The vinylaromatic polymers are cross-linked, preferably by copolymerization with cross-linking monomers having more than one, preferably 2 or 3, copolymerizable Cxe2x95x90C. double bond(s) per molecule. Such cross-linking polymers include, for example, polyfunctional vinylaromatics such as di- and trivinylbenzenes, divinylethylbenzene, divinyltoluene, divinylxylene, divinylnaphthalene, polyfunctional allyl aromatics such as diallylbenzenes and triallylbenzenes, cyanurates and isocyanurates such-as trivinyl cyanurate, triallyl cyanurate, trivinyl isocyanurate and triallyl isocyanurate, N,Nxe2x80x2-C1-C6-alkylenediacrylamides and -dimethacrylamides such as N,Nxe2x80x2-methylenediacryl-amide and -dimethacrylamide, N,Nxe2x80x2-ethylenediacrylamide and -dimethacrylamide, polyvinyl ethers and polyallyl ethers of saturated C2-C20-polyols having 2 to 4 OH groups per molecule, such as ethylene glycol divinyl ether and ethylene glycol diallyl ether and diethylene glycol divinyl ether and diethylene glycol diallyl ether, esters of acrylic and methacrylic acid with unsaturated C3-C12-alcohols or of saturated C2-C20-polyols having 2 to 4 OH groups per molecule, such as allyl methacrylate, ethylene glycol di(meth)acrylate, glycerol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate and divinylethyleneurea and divinylpropyleneurea, divinyl adipate, aliphatic and cycloaliphatic olefins having 2 or 3 isolated Cxe2x95x90C. double bonds, such as hexa-1,5-diene, 2,5-dimethylhexa-1,5-diene, octa-1,7-diene, 1,2,4-trivinylcyclohexane. Cross-linking monomers which have proved to be particularly useful are divinylbenzene (as mixture of isomers) and mixtures of divinylbenzene and aliphatic C6-C12-hydrocarbons containing 2 or 3-Cxe2x95x90C. double bonds. The cross-linking monomers are generally used in amounts of 2 to 20% by weight, preferably 2 to 12% by weight, based on the total amount of the polymerizable monomers used.
The cross-linking monomers need not be used in pure form, but can also be used in the form of their industrially handled mixtures of lower purity (eg. divinylbenzene in mixtures with ethylstyrene).
For the preparation of the weakly basic anion exchangers from the cross-linked vinylaromatic polymers, the chloromethylation process or the aminomethylation processxe2x80x94in each case followed by subsequent aminationxe2x80x94can be used.
By the chloromethylation process, chloromethyl group-containing cross-linked vinylaromatic polymers are prepared, which are then reacted with non-tertiary amines or polyamines, eg. the reaction of cross-linked polystyrene with monochlorodimethyl ether and subsequent amination with dimethylamine.
In contrast, according to the aminomethylation process, cross-linked vinylaromatic polymers are reacted with reactive phthalimide derivatives, such as N-chloromethyl-phthalimide, and the resulting imides are hydrolyzed to give the corresponding primary amines. These primary amino groups can then themselves be reacted with customary alkylating agents, such as methyl chloride, or can be reacted by Leuckart-Wallach reductive alkylation using carbonyl compounds and formic acid as reducing agent.
The cross-linked vinylaromatic polymers bearing tertiary amino groups represent the starting materials for the process according to the invention. They preferably contain 0.3 to 2 amino groups per aromatic nucleus. The starting materials can be in gel form or macroporous.
It is advisable to suspend the weakly basic anion exchanger (=starting material) in water in the tertiary amine form with stirring. Generally, the water is heated to the intended reaction temperature already prior to the addition of the starting material or, alternatively, the suspension of the starting material in water is heated to the intended reaction temperature. Ethylene oxide is then introduced in stages orxe2x80x94preferablyxe2x80x94as far as possible continuously at a rate which permits reliable temperature control. The pH is continuously controlled during this but kept in the range according to the claims by adding a suitable inorganic strong acid, for example sulphuric acid, phosphoric acid, hydrochloric acid and those acids having a pka  less than 2, more precisely preferably during the entire course of the reaction. The total amount of the acid to be added is generally 0.4 to 0.6, preferably 0.48 to 0.52, mol, based on amino groups in the starting material.
If it is wished to convert the tertiary amino groups as quantitatively as possible to quaternary ammonium groups, it is advisable to use up to 3, preferably 1.05 to 1.5, mol of ethylene oxide per mol of tertiary amino groups. After the ethylene oxide addition is completed, a certain extra time is allowed for further reaction. The progress of the reaction may be followed in this case by analyzing samples taken off. After completion of the reaction, residual ethylene oxide is taken off, the reaction product is washed with water and, if appropriate, converted to the chloride, hydroxide, sulfate form using aqueous common salt solution. By means of the process according to the invention, degrees of conversion of the tertiary amino groups to quaternary ammonium groups of greater than 95, preferably greater than 98, and in particular greater than 99,% may be achieved.
The percentages in the following examples are all based on weight.